If you are commenting on a student paper, editing, or writing, you might like the Owl Purdue site as a reference. It is easy to search. It is also a reliable source of information on many issues related to writing.

An example is how the site provides an overview of the use of the comma. I feel I can always use a refresher on the proper use of a comma and think maybe we all could use a refresher. So, let’s see what the site has to say about what should be our friend – the comma. The following comes from the https://owl.purdue.edu/owl/general_writing/punctuation/punctuation/index.html site.

Comma

Use a comma to join two independent clauses and a coordinating conjunction (and, but, or, for, nor, so).

Road construction can be inconvenient, but it is necessary.
The new house has a large fenced backyard, so I am sure our dog will enjoy it.

Use a comma after an introductory phrase, prepositional phrase, or dependent clause.

To get a good grade, you must complete all your assignments.
Because Dad caught the chickenpox, we canceled our vacation.
After the wedding, the guests attended the reception.

Use a comma to separate elements in a series. Although there is no set rule that requires a comma before the last item in a series, it seems to be a general academic convention to include it. The examples below demonstrate this trend.

On her vacation, Lisa visited Greece, Spain, and Italy.
In their speeches, many of the candidates promised to help protect the environment, bring about world peace, and end world hunger.

Use a comma to separate nonessential elements from a sentence. More specifically, when a sentence includes information that is not crucial to the message or intent of the sentence, enclose it in or separate it by commas.

John’s truck, a red Chevrolet, needs new tires.
When he realized he had overslept, Matt rushed to his car and hurried to work.

Use a comma between coordinate adjectives (adjectives that are equal and reversible).

The irritable, fidgety crowd waited impatiently for the rally speeches to begin.
The sturdy, compact suitcase made a perfect gift.

Use a comma after a transitional element (however, therefore, nonetheless, also, otherwise, finally, instead, thus, of course, above all, for example, in other words, as a result, on the other hand, in conclusion, in addition)

For example, the Red Sox, Yankees, and Indians are popular baseball teams.
If you really want to get a good grade this semester, however, you must complete all assignments, attend class, and study your notes.

Use a comma with quoted words.

“Yes,” she promised. Todd replied, saying, “I will be back this afternoon.”

Use a comma in a date.

October 25, 1999
Monday, October 25, 1999
25 October 1999

Use a comma in a number.

15,000,000
1614 High Street

Use a comma in a personal title.

Pam Smith, MD
Mike Rose, Chief Financial Officer for Operations, reported the quarter’s earnings.

Use a comma to separate a city name from the state.

West Lafayette, Indiana
Dallas, Texas